1. Field of the Invention
The present invention relates to electroless deposition of metal on surfaces where catalytic metal is supported by a carrier-polymer that can withstand high temperatures and stresses of extrusion processes and laundering.
2. Description of the Prior Art
In electroless deposition processes an article is coated with a metal film without the application of an electric current. Typically, a surface containing a noble metal such as palladium catalytically initiates deposition of a metal such as nickel, copper or cobalt from a solution containing a chemical reducing agent. Once initiated, electroless deposition is autocatalytic in that deposited reduced metal provides an expanding catalytic surface for further deposition. Such techniques are commonly used to deposit metal on plastic or resinous substrates, e.g. to produce printed circuit boards, laser or magnetic data storage devices, catalytic devices, electromagnetic shielding of electronic equipment housings, conductive coatings, decorative coatings, antistatic coatings and the like.
Considerable effort in the art of electroless depositions has been devoted to improving the quality of electrolessly-deposited metal coatings. U.S. Pat. No. 3,414,427 discloses that better adhesion of metal coatings is achieved by use of a more soluble complex, e.g., of palladium chloride, hydrogen chloride and water. Other developments based on modified palladium complexes are disclosed in U.S Pat. Nos. 3,520,723 (cuprous iodide treatment), 3,847,648 (ketopalladium complexes), 3,937,857 and 4,006,047 (thermo-decomposable palladium complexes) and 3,963,841 (dimethyl sulfoxide complexes).
Other attempts to improve metal coating adhesion have included etching of polymeric substrates, e.g., with chromic and/or sulfuric acid. See, for instance, U.S. Pat. Nos. 3,370,974; 3,423,226; 3,437,507; 3,507,681; 3,515,649; 3,616,296; and 3,702,286 which disclose various acid etching techniques which are often useful in preparing surfaces comprising ABS (a multi-phase thermoplastic of disbursed butadiene with grafted styrene acrylonitrile copolymer).
Polymers used with catalytic metal compounds in electroless deposition applications are referred to herein as "carrier polymers" in the sense that they form a polymeric coating which carries on its surface or presents the catalytic metal to the deposited metal species in the plating bath. In this regard see U.S. Pat. No. 4,910,072 for electroless deposition on substrates coated with a polymer, e.g. polybutadiene or polyvinyl chloride, complexed with a catalytic compound and activated, for example, by heating. See also U.S. Pat. No. 5,082,734 for a disclosure of electroless deposition on substrates coated with an aqueous mixture, e.g. solution or emulsion, of a polymer and a catalytic compound; exemplary coatings are prepared from solutions of water soluble polymers such as polyvinyl alcohol, modified cellulose or emulsions of water-insoluble polymers such as a polyethylene latex. Bright, shiny electroless deposition of copper is illustrated by the use of the polymers disclosed in U.S. Pat. Nos. 5,082,734 and 4,910,072 which contain only carbon and, optionally, oxygen in the backbone. The use of such polymers, however, has been found to suffer from a number of drawbacks; for instance, such polymers generally are sufficiently hydrophilic that they tend to swell and/or dissolve in the aqueous plating solution, therefore leading to incomplete metal coating and to bath "crashing". That is, some or all of the catalytic compound with or without an attendant portion of carrier polymer can be washed from the substrate surface in the agitation of the plating bath, causing depletion of the metal value of the plating bath as uncontrolled metal deposition occurs. Moreover, the limitations on the molecular structures permissible for these hydrophilic carrier polymers in turn places constraints on a number of characteristics, including thermal stability. This has been found to be a particularly undesirable characteristic when metal-coated fibers are extruded to form shaped articles, such as equipment housings.
Moreover, the requirement that the carrier polymer be suitable for aqueous application places limitations on the thermal stability of the carrier polymer since highly hydrophilic polymers soften excessively or decompose at relatively low temperatures. This has been found to be a particularly undesirable characteristic when metallized fibers are subjected to heat and shear stresses when they are incorporated into the feed mixture of an extruder or injection molding machine. Presumably, the heat and shear stresses encountered by a metal-coated fiber as it passes through a molding machine tends to soften, melt or decompose the carrier polymer thereby weakening or destroying the bond between the metal coating and the substrate. When hydrophilic polymers are used as carrier polymers, the resultant metallized fibers loose virtually all of their metal coating when they are used in injection molding feed mixtures (even when they are present at concentrations of 1 volume percent or less). It is also believed that the loss of conductivity and electromagnetic shielding that occurs when plated fabrics prepared by aqueous application of the carrier polymer are laundered is also associated with the softening of a hydrophilic polymer in the presence of water and heat.